When a helicopter is flying horizontally, or hovering in the wind, differing relative wind speeds cause the rotating blades to experience differing horizontal forces throughout each rotation. For example, during forward flight, when the blade is advancing it is encountering a larger relative air speed than when the blade is retreating. Accordingly, each blade experiences large and varying moments in the leading and lagging directions. Rather than rigidly attaching blades to a yoke and forcing the yoke to absorb the large varying moments, the blades may be attached to the yoke via a lead-lag hinge which has an axis of rotation substantially parallel to the mast axis. In order to prevent the blades from rotating too far back and forth about the lead-lag hinge, and to prevent the back and forth movement from matching the resonant frequency of the drive system, dampers may be attached to the blades to provide a resistive force.
The blades also experience large forces in a direction parallel to the lead-lag hinge axis. In order to allow some movement in this direction, a flap hinge may be utilized. The flap hinge attaches the blades to the yoke about an axis perpendicular to the lead-lag hinge axis.
In addition to the optional lead-lag and flap hinges, the blades must be able to collectively and cyclically alter their pitch to enable vertical and horizontal movement of the helicopter. Therefore, each blade must be hinged about a pitch change axis that is generally perpendicular to both the lead-lag hinge and flap hinge axes.
The dampers may be coupled between the blades and the yoke or they may be coupled between adjacent blades, known as blade-to-blade dampers. Blade-to-blade dampers have generally been attached proximate the trailing end of one blade grip and to the leading end of the adjacent blade grip. As such, the attachment points of the dampers are laterally offset from the pitch change axis. When the blades are rotated away from horizontal, any resistive force applied by the damper to the blade causes a rotational moment about the pitch change axis. This moment must be resisted by the flight control system in order to maintain the desired blade pitch. As the blade rotates about the pitch change axis, the effective length of the lever arm changes, and therefore, so does the moment. This is further complicated by the constantly changing resistive force which also modifies the magnitude of the moment. These constantly changing moments unnecessarily complicate the dynamic analysis required to effectively design and program the flight control system.